Grating based optical coupler

ABSTRACT

An apparatus including a waveguide region configured to guide light propagating along a first direction; a reflector region configured to reflect incident light; an interference region formed between the waveguide region and the reflector region, the interference region configured to confine at least a portion of interference light formed by the incident light and the reflected incident light; and a grating region including a grating formed on a region confining at least a portion of the interference light, the grating configured to couple at least a portion of the light along a second direction that is different from the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional Patent Application No. 62/014,182, filed Jun. 19, 2014, U.S. Provisional Patent Application No. 62/012,446, filed Jun. 16, 2014, U.S. Provisional Patent Application No. 61/979,489, filed Apr. 14, 2014, U.S. Provisional Patent Application No. 61/946,657, filed Feb. 28, 2014, U.S. Provisional Patent Application No. 61/925,629, filed Jan. 9, 2014, and U.S. Provisional Patent Application No. 61/895,493, filed Oct. 25, 2013 which are incorporated by reference herein.

BACKGROUND

This specification relates to coupling light using a grating.

Light propagates inside a photonic integrated circuit, and is coupled to an external medium through a grating fabricated on the photonic integrated circuit.

SUMMARY

According to one innovative aspect of the subject matter described in this specification, light may be generated, guided, processed, or detected by one or more active and/or passive optical components of a photonic integrated system. A grating may be formed on an interference region of the photonic integrated circuit, where the interference region may be bounded by one or more reflectors. By matching a grating profile with an interference pattern produced by the one or more reflectors, light may reciprocally enter or exit the photonic integrated circuit at a specific angle from or to an external medium, respectively.

In general, one innovative aspect of the subject matter described in this specification can be embodied in an optical apparatus that includes a waveguide region configured to guide light propagating along a first direction; a reflector region configured to reflect incident light; an interference region formed between the waveguide region and the reflector region, the interference region configured to confine at least a portion of interference light formed by the incident light and the reflected incident light; and a grating region including a grating formed on a region confining at least a portion of the interference light, the grating configured to couple at least a portion of the light along a second direction that is different from the first direction.

This and other implementations can each optionally include one or more of the following features. The second direction may be substantially perpendicular to the first direction. The waveguide region may include a second reflector region configured to (i) reflect, by a particular reflectivity, a portion of incident light, and (ii) transmit another portion of the incident light. Light may be attenuated based on an one-circulation attenuation coefficient after propagating one-circulation in the interference region. The reflectivity of the second reflector region may be configured to substantially match the one-circulation attenuation coefficient. A portion of the grating region may be formed on the second reflector region.

The optical apparatus may include an n-doped region and a p-doped region configured to provide an electric field in the second reflector region with an application of a voltage or a current across the n-doped region and the p-doped region, where the second reflector region may be configured to provide a different reflectivity with the application of a voltage or a current across the n-doped region and the p-doped region.

The optical apparatus may include an n-doped region and a p-doped region configured to provide an electric field in the reflector region with an application of a voltage or a current across the n-doped region and the p-doped region, where the reflector region may be configured to provide a different reflectivity with the application of a voltage or a current across the n-doped region and the p-doped region.

The optical apparatus may include an n-doped region and a p-doped region configured to provide an electric field in the interference region with an application of a voltage or a current across the n-doped region and the p-doped region, where the interference region may be configured to provide a different interference pattern for the interference light with the application of a voltage or a current across the n-doped region and the p-doped region.

The optical apparatus may include a light source region configured to generate light, wherein the light source region is coupled to the interference region, the waveguide region, or the reflector region. The optical apparatus may include an optical package having an aperture above the grating region, the aperture configured to transmit light along the second direction.

The optical apparatus may include a light absorption region configured to absorb at least a portion of light, wherein the light absorption region is coupled to the interference region, the waveguide region, or the reflector region.

The grating region may include a two-dimensional grating configured to separate light into two light portions propagating along two directions, or combine two light portions propagating along two directions into one light portion.

A second grating region may be formed on a side of the interference region that is at a different side to the grating region, the second grating region configured to couple at least a portion of the light along a third direction. The reflector region may include one of a corner mirror, a DBR mirror, a dispersive mirror, a waveguide loop mirror, or a metal layer. A grating periodicity of the grating region may substantially match an interference periodicity of the light inside the interference region. The grating may have lattice vectors formed so that the locations of the in-phase antinodes of the light inside the interference region substantially match the locations of the grating valleys or peaks. A portion of the grating region may be formed on the interference region, the reflector region, or the waveguide region.

Another innovative aspect of the subject matter described in this specification can be embodied in an optical apparatus that includes a light source region configured to generate photons; a first reflector region and a second reflector region; an interference region formed between the first reflector region and the second reflector region and coupled to the light source region, and the interference region configured to (i) guide the incident light generated by the light source region to propagate along a first direction, and (ii) confine interference light formed by light reflected between the first reflector region and the second reflector region along the first direction; and a grating region including a grating formed on a region confining at least a portion of the interference light, the grating configured to emit a portion of the interference light along a second direction that is different from the first direction.

This and other implementations can each optionally include one or more of the following features. The effective refractive index of the interference region may be lower than the effective refractive index of the grating region.

Another innovative aspect of the subject matter described in this specification can be embodied in an optical apparatus that includes a first reflector region and a second reflector region; an interference region formed between the first reflector region and the second reflector region, the interference region configured to confine interference light formed by light reflected between the first reflector region and the second reflector region propagating along a first direction; and a light absorption region coupled to the interference region, the light absorption region configured to absorb at least a portion of the interference light; and a grating region comprising a grating formed on a region confining at least a portion of the interference light, the grating configured to receive, from an external medium, light propagating along a second direction that is different from the first direction.

This and other implementations can each optionally include one or more of the following features. The light absorption region and the grating region may have different material composition, where the grating region may include metal forming surface plasmonic effect that couples light propagating along the second direction to the light absorption region.

Advantageous implementations may include one or more of the following features. Light may be coupled in or out of a photonic integrated circuit at an angle that is substantially perpendicular to the propagation direction of the light inside the photonic integrated circuit. This perpendicularity may reduce packaging cost and complexity. Reflected light back to the photonic integrated circuit may be minimized by one or more mirrors in the interference region to maintain stability of the photonic integrated circuit. The optical mode profile of the light exiting the grating may be shaped to match the optical mode profile of the external medium to minimize mode matching loss. Since the electric field in the interference region is quite uniform, there is no need for a chirped grating to match a traveling wave caused exponentially-decayed field profile. The interference region or grating may be actively tuned by mechanisms including electrical field, magnetic field or mechanical movement, to control the coupling of light. When the interference region is coupled to an active medium that generates light at a broad wavelength range, the interference region may be used to select a narrower wavelength range from the broad wavelength range. When the interference region is coupled to an absorption medium that detects light, the interference region may be used to increase the absorption efficiency through multiple reflections inside the interference region.

Other implementations of this and other aspects include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices. A system of one or more computers can be so configured by virtue of software, firmware, hardware, or a combination of them installed on the system that in operation cause the system to perform the actions. One or more computer programs can be so configured by virtue of having instructions that, when executed by data processing apparatus, cause the apparatus to perform the actions.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other potential features and advantages will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an example photonic integrated circuit.

FIGS. 1B, 1C, and 1D show examples of an optical coupler.

FIG. 2 is an example of interference patterns.

FIGS. 3A-3E show examples of a grating pattern.

FIGS. 4A-4F show examples of an optical coupler integrated with a light source.

FIGS. 5A-5C show examples of an optical coupler integrated with a light detector.

FIG. 6 shows an example of an optical coupler integrated with p-n junctions.

FIGS. 7A-7B show examples of an optical coupler having multiple output paths.

FIG. 8 shows an example of an optical coupler for packaging.

FIGS. 9A-9E show examples of a mirror.

FIG. 10 shows an example of a flow diagram for designing an optical coupler.

FIG. 11 shows an example of a flow diagram for fabricating an optical coupler.

Like reference numbers and designations in the various drawings indicate like elements. It is also to be understood that the various exemplary embodiments shown in the figures are merely illustrative representations and are not necessarily drawn to scale.

DETAILED DESCRIPTION

FIG. 1A is a block diagram of an example photonic integrated circuit 100 that includes an grating-based optical coupler for enabling the coupling of light into and out of a photonic integrated circuit 100. In general, an optical coupler with substantial vertical emission is useful for interfacing surface-emitting/receiving optoelectronic devices, and can reduce the packaging cost and complexity due to off-normal configuration.

The photonic integrated circuit 100 includes one or more optical components fabricated on a substrate 116. The optical components include a waveguide region 102, a first reflector region 106, an interference region 110, a second reflector region 114, and a grating region 120. The substrate 116 may be any type of substrate that is suitable for fabricating a photonic integrated circuit. For example, the substrate 116 may be a silicon wafer, a silicon-on-insulator (SOI) wafer, a III-V material such as a gallium arsenide (GaAs) or an indium phosphide (InP) wafer, or a glass wafer. As another example, the substrate 116 may be a layer of passive or active material deposited over an integrated electronic circuit. As another example, the substrate 116 may be a layer of passive or active material deposited over another integrated photonic circuit.

In general, the waveguide region 102 is configured to confine light along one or more dimensions to guide light along a specific direction. In some implementations, the waveguide region 102 may confine light along one dimension. For example, the waveguide region 102 may be a slab waveguide that confines light along the z direction. In some implementations, the waveguide region 102 may confine light along two dimensions. For example, the waveguide region 102 may be a rib waveguide or a channel waveguide that confines light along the y and z directions, such that the light may propagate along the x direction, as designated by the arrow 122. The term “along the x direction” and their derivatives as used herein may be used to represent bidirectional (±x direction), or unidirectional (+x, −x). Furthermore, when light is traveling inside a multi-mode fiber placed along the x direction, some portions of the light may travel in zigzag routes inside the fiber while the overall direction may still be regarded as along the x direction.

In general, the first reflector region 106 and the second reflector 114 are configured to reflect incident light. For example, when light in the waveguide region 102 is incident on an interface 104, some portion of the light may be reflected back to the waveguide region 102, while the remaining portion of the light may be transmitted to the first reflector region 106. Similarly, when light in the first reflector region 106 is incident on an interface 108, some portion of the light may be reflected, while the remaining portion of the light may be transmitted to the interference region 110. Similarly, when light in the interference region 110 is incident on an interface 112, some portion of the light may be reflected, while the remaining portion of the light may be transmitted to the second reflector region 114. In some implementations, a reflector may be an interface between two mediums with different refractive indices.

The portion of the light that is reflected by a reflector may range from near zero percent to near one hundred percent, depending on the design. In some implementations, the first reflector region 106 or the second reflector 114 may be highly reflective. For example, the second reflector 114 may be coated with a metal, such as aluminum, to achieve a high reflectivity. As another example, light may be arrange to be incident on the second reflector 114 beyond a critical angle, where the light is reflected through total internal reflection. As another example, the second reflector 114 may be a Bragg reflector that provides high reflectivity for a range of wavelengths. As another example, the first reflector region 106 may comprise one or multiple slits disconnecting the waveguide region 102 and the interference region 110. As another example, the first reflector region 106 may comprise DBR structure. As another example, the first reflector region 106 may be an anomalous dispersive mirror having multiple high/low refractive index structures in the x direction to compensate the wavelength-dependent optical phase of the interference region and achieve a wider bandwidth of operation.

In some implementations, the first reflector region 106 or the second reflector 114 may be partially transmissive and partially reflective. For example, the first reflector region 106 may be configured to (i) reflect, by a particular reflectivity, a portion of incident light, and (ii) transmit another portion of the incident light. A partial reflective reflector may be implemented, for example, by depositing a dielectric material having a refractive index that is lower than the material of the waveguide region 102 in the corresponding reflector region. The percentage of reflected and transmitted light may be calculated using Fresnel equations.

In general, the interference region 110 acts as a cavity having a cavity length, L_(Cavity), that is formed between the waveguide region 102 and the second reflector region 114. In some implementations, the first reflector region 106 may be formed between the waveguide region 102 and the interference region 110, where L_(Cavity) may be defined as the length between the first reflector region 106 and the second reflector region 114. In some implementations, an effective refractive index of the waveguide region 102 may be substantially equal to an effective refractive index of the interference region 110. For example, both the waveguide region 102 and the interference region 110 may be fabricated in silicon with the same cross sectional waveguide dimensions along the y-z dimensions. In this case, the effective refractive index of the waveguide region 102 is equal to an effective refractive index of the interference region 110. As another example, both the waveguide region 102 and the interference region 110 may be fabricated in silicon, but the cross sectional waveguide dimensions along the y-z dimensions may vary, which may cause a difference between the effective refractive index of the waveguide region 102 and the effective refractive index of the interference region 110. In this case, the effective refractive index of the waveguide region 102 is treated as substantially equal to the effective refractive index of the interference region 110 as long as a resulting performance degradation, such as an optical loss, caused by the effective refractive index difference is within a range that is acceptable for a targeted application.

The interference region 110 is configured to confine an interference light formed by an incident light and a reflected incident light. For example, a standing wave pattern between the first reflector 106 and the second reflector 114 may be formed in the interference region 110. To have interference formed in the interference region 110, the length of the cavity L_(Cavity) and the parameters of grating region 120 are chosen so that the incident light may reach to and get reflected by the second reflector 114 without being totally attenuated in the first pass propagating from the first reflector 106 to the second reflector 114. In some implementations, the confinement may be a partial confinement, where a portion of the interference light is transmitted through the first reflector 106 back to the waveguide region 102 and/or a portion of the interference light is transmitted through the second reflector 114. The interference of light formed by the incident light and the reflected incident light is described in more details in FIG. 2.

In some implementations, an optical path length of the interference region 110 may be longer than a wavelength of the guided light. In some other implementations, an optical path length of the interference region 110 may be shorter than the wavelength of the guided light. For example, for an interference region 110 composed of silicon having a cavity length of 0.4 μm and a refractive index of 3.45, the interference region 110 has an optical path length of 0.4 μm×3.45=1.38 μm. If the wavelength of the guided light has a wavelength of 1.55 μm, the optical path length of the interference region 110 is shorter than the wavelength of the guided light. In this case, the light having the 1.55 μm wavelength may be coupled to the grating region 120 through an evanescent field of the light confined (partial confinement) in the interference region 110.

In general, the grating region 120 having a grating length of, L_(Grating), is configured either to couple at least a portion of the light in a photonic integrated circuit 100 to an external medium 130, or to couple at least a portion of the light from the external medium 130 to the photonic integrated circuit 100. In some implementations, the grating length L_(Grating) may be shorter than the cavity length L_(Cavity). In some other implementations, the grating length L_(Grating) may be equal to the cavity length L_(Cavity). In some other implementations, the grating length L_(Grating) may be longer than the cavity length L_(Cavity). For example, the grating region 120 may be fabricated on the interference region 110, but a portion of the grating region 120 may be extended into the waveguide region 102, and/or the first reflector region 106, and/or the second reflector region 114. As used in this specification, the grating is formed or fabricated on a region means that the grating is formed above the region, or the grating is embedded at least partially inside the region. For example, the grating may be formed by etching into the region which the grating is disposed over.

In some implementations, the interference region 110 and the grating region 120 may have the same material composition. For example, the grating region 120 may be fabricated by etching a grating pattern directly on the surface of the interference region 110. In some other implementations, the interference region and the grating region may have different material compositions. For example, the grating region 120 may be fabricated by depositing silicon dioxide on the surface of a silicon-based interference region 110. A grating pattern may then be etched on the surface of the silicon dioxide to form an oxide grating. As another example, the grating region 120 may be fabricated by depositing metal on the surface of the interference region 110 and then be etched to form a metal grating. As another example, the grating region 120 may be fabricated by depositing a higher refractive index material on the surface of the interference region 110 having a lower refractive index to improve the grating efficiency by attracting the optical mode toward the grating side. The lower refractive index material can for example be InP and the higher refractive index material can for example be Si.

In general, the grating region 120 redirects light propagating along a first direction to a second direction that is different from the first direction. In some implementations, the grating region 120 may redirect light propagating along a first direction to a second direction that is substantially perpendicular to the first direction. For example, by substantially matching a grating periodicity of a grating region 120 to the interference periodicity of a standing wave pattern in the interference region 110, the grating region 120 may redirect the light propagating inside the waveguide region 102 along the x direction, as designated by the arrow 122, to a perpendicular direction along the z direction, as designated by the arrow 123. The term “substantially match” as used in this application means that a resulting performance degradation, such as an optical loss, due to the mismatch is within an acceptable range for a targeted application. An acceptable range may be, for example, within an order of magnitude. In some other implementations, the grating region 120 may redirect light propagating along the first direction to a second direction that is not substantially perpendicular to the first direction. The term “substantially perpendicular” as used in this application means 90 degrees with an error margin that is acceptable for a targeted application.

The external medium 130 may be any medium that can transmit, guide, detect, or generate light. For example, the external medium 130 may be an optical fiber. As another example, the external medium 130 may be a photo-detector. As another example, the external medium 130 may be a light source. In some implementations, a cladding 124 may be formed between the grating region 120 and the external medium 130. The cladding 124 may be formed to protect the photonic integrated circuit 100, or to provide a specific distance between the grating region 120 and the external medium 130. In some implementations, a cross-sectional mode profile of the light emitted from the grating region 120 may be designed to substantially match a cross-sectional mode profile of the external medium 130 configured to receive the light emitted from the grating region. For example, the cross-sectional mode profile of the light emitted from the grating region 120 in the x-y dimensions may be designed to substantially match the cross-sectional mode profile of the single mode fiber in the in the x-y dimensions.

FIG. 1B shows an example of an optical coupler 101 that may be implemented in the photonic integrated circuit 100. The optical coupler 101 may also be implemented in any one of the other photonic integrated circuits described throughout this application, or in another photonic integrated circuit not described in this application.

The optical coupler 101 includes an interference region 110 and a grating region 120. The grating region 120 includes grating valleys 118 and grating peaks 126, which together form a grating having a grating length, L_(Grating). The height difference between a grating peak 126 and a grating valley 118 determines a grating height. The ratio of the peak width to the sum of peak width and valley width of the grating along the wave propagation direction determines the duty cycle of the grating. The sum of the grating peak width and the grating valley width determines the period of the grating. By tuning the grating height, the duty cycle, the grating period, the grating shapes, the cladding covering the grating, or a combination of the above, the directionality and far field angle of light emitted/received by the grating region 120 can be determined. For example, the grating height and the duty cycle may be modified to optimize the directionality of the light. As another example, the grating period and the duty cycle may be tuned to achieve the desired far field angles which might be most suitable for a targeted application.

In some implementations, the height of the grating peaks may be higher than the height of the first reflector region 106 and/or the second reflector region 114. For example, the first reflector region 106, the interference region 110, and the second reflector region 114 may be planarized by polishing, and then another layer of material is deposited on the planarized surface so that the grating region 120 may be formed by patterning and etching.

In some other implementations, the height of the grating valleys may be lower than the height of the first reflector region 106 and/or the second reflector region 114. FIG. 1C shows an example of an optical coupler 103 where the height of the grating valleys 119 is lower than the height of the first reflector region 106 and the second reflector region 114. For example, the first reflector region 106, the interference region 110, and the second reflector region 114 may be planarized by polishing, and then the grating region 120 may be formed on the polished surface by patterning and etching the interference region 110. The optical coupler 103 may be implemented in the photonic integrated circuit 100. The optical coupler 103 may also be implemented in any one of the other photonic integrated circuits described throughout this application, or in another photonic integrated circuit not described in this application.

FIG. 1D shows an example of an optical coupler 105 that includes the waveguide region 102, the interference region 110, the grating region 120, and the second reflector region 114, but does not include the first reflector region 106. The boundary 130 between the waveguide region 102 and the interference region 110 is denoted by a dashed line 130 because in some implementations, the waveguide region 102 and the interference region 110 are composed of the same material or having substantially equal effective refractive index.

The optical coupler 105 does not include the first reflector region 106 in the case where light in the interference region 110 is attenuated below a threshold value after propagating one-circulation in the interference region 110. For example, a standing wave may be created in the interference region 110 through interference between forward light incident on the second reflector region 114 and backward light reflected by the second reflector region 114. The standing wave may diminish near the boundary 130 between the waveguide region 102 and the interference region 110 because the light reflected by the second reflector region 114 is attenuated below the threshold value beyond the boundary 130. The threshold value, for example, may be less than 10% of the initial incident light power. By substantially matching a grating pattern in the grating region 120 with the standing wave pattern in the interference region 110, the optical coupler 105 can be used to redirect light propagating along a first direction to a second direction that is different to the first direction without the first reflector region 106. For example, the optical coupler 105 may be used to direct light to a second direction that is substantially perpendicular to the first direction. In some implementations, without the first reflector region 106, the optical coupler can still redirect an incident light with high efficiency if the light is attenuated below a threshold value after propagating one-circulation in the interference region 110. In some implementations, to maintain high efficiency without introducing the first reflector region 106, the grating region 120 needs to provide enough one-circulation attenuation. For example, the grating length L_(Grating) needs to be long enough to provide sufficient one-circulation attenuation before reaching the boundary 130.

FIG. 2 is an example of a grating pattern 207 that substantially matches an interference periodicity of the standing wave 205 inside the interference region. The descriptions of FIG. 2 may be applied to any one of the optical couplers described in this application. Generally, the round-trip phase shift is the sum of the phase shift introduced by the one-circulation propagation and the phase shift introduced by a reflector. To simplify the description, the phase shift introduced by the reflector may be assumed to be zero so that the resonant condition “round-trip phase shift equals 2 mπ” can be viewed as the same as “one-circulation phase shift equals 2 mπ” where m is any integer.

In some implementations, light propagating in a waveguide may be confined by two dimensions. For example, referring to FIG. 1A, light propagating in the waveguide region 102 is confined in y and z dimensions. When the light enters an interference region, the confinement of the waveguide may be weakened, and the light propagates like a point wave inside the interference region. For example, the interference region 110 may be designed to confine light tightly in the z dimension and confine light loosely in the y dimension. The point wave reaches a reflector 211, gets reflected, and a standing wave pattern 205 having a periodicity of d1 may be formed inside the interference region through the interference of a forward propagating wave 201 and a backward propagating wave 203.

In some implementations, a grating pattern 207 may be designed to substantially match the standing wave pattern 205. By matching the standing wave pattern 205, this grating pattern 207 may act as an optical antenna and become the most efficient way for light to leave the interference region. Each grating structure may act to emit the light as a point wave, and all point-wave fronts emitted from individual grating structures are combined into a planar wave front which propagates in a perpendicular direction with low loss. For example, one theoretical condition for an ideal match may be d2=2×d1.

Based on the material quality and the physical dimensions of the interference region and grating structure, an one-circulation attenuation coefficient, α, may be calculated along with a corresponding phase shift for the resonant condition inside the interference region. For example, the interference region may be composed of a material that has a specific absorption coefficient for the guided light, which contributes to the one-circulation attenuation coefficient. As another example, light may be emitted by the grating region during propagation, which also contributes to the one-circulation attenuation coefficient. In general, light is attenuated based on an one-circulation attenuation coefficient after propagating one-circulation in the interference region (i.e., forward propagation from the boundary 213 to the reflector 211, and then backward propagation from the reflector 211 to the boundary 213). The term “one-circulation attenuation coefficient α” as used in this application refers to a ratio between the remaining optical power after the one-circulation attenuation and the initial optical power.

In some implementations, to substantially reduce back reflection loss, a reflector region (e.g., the first reflector region 106) may be placed at the boundary 213, where the reflectivity of the reflector region at the boundary 213 is configured to substantially match the one-circulation attenuation coefficient α. By substantially matching the reflectivity of the reflector region at the boundary 213 with α, the light (from the left-hand side of 213) reflected from the boundary 213 back to the incident light source (at the left-hand side of 213) and the light (from the right-hand side of 213) transmitting through the boundary 213 back to the incident light (at the left-hand side of 213) cancel with each other after multiple passes due to destructive interference, meaning almost all the power of the original incident light (incident from the left of 213 into the region between 213 and 211) is transferred into the region between 213 and 211. In some implementations, the one circulation attenuation coefficient α may be near zero. In this case, the corresponding reflectivity r at the boundary 213 can be set as zero, which corresponds to the optical coupler 105 in FIG. 1D, where the first reflector region 106 is not included in the optical coupler 105. In some implementations, the reflectivity r at the boundary 213 can be set as high as the reflectivity of the reflector 211, (e.g., close to 1), to form a highly confined cavity along the x direction, where the light can enter or leave the cavity through another direction such as in the z direction.

In some implementations, there may be non-ideal factors that affect the performance. For example, a change of effective refractive index may occur from the etching of the grating region on the interference region. As another example, the etching process may not create a straight line from the grating peak to the grating valley. While the theoretical matching condition is d2=2d1, slight deviation from the exact condition may be expected during real implementation. Such deviation does not change the functionality of the optical coupler, but may affect the efficiency. However any reasonable deviation from the ideal case is within the scope of this disclosure, where the efficiency is acceptable for a targeted application. An iterative process of fabricating the optical coupler, testing the optical coupler, and then re-designing the optical coupler may improve this issue.

FIG. 3A shows an example of a view of a grating pattern 331 on a plane along the x-y dimensions. The descriptions of FIG. 3A may be applied to any one of the optical couplers described in this application. The grating pattern 331 includes an array of one-dimensional grating structures 301 a-n and 303 a-n along the x direction, where n is any integer greater than one. In some implementations, the grating structures 301 a-n and 303 a-n may be composed of different materials. For example, the grating structures 301 a-n may be composed of silicon, and the grating structures 303 a-n may be composed of InP. As another example, the grating structures 303 a-n may include a layer of metal that forms surface plasmonic effect that couples light from an external medium to the interference region. The arrangement of 301 a, 303 a, 301 b, 303 b, . . . , 301 n, and 303 n forms the grating in a grating region.

FIG. 3B shows an example of a view of a grating pattern 332 on a plane along the x-y dimensions. The descriptions of FIG. 3B may be applied to any one of the optical couplers described in this application. The grating pattern 332 includes an array of one-dimensional grating structures 305 a-n along the x direction, where n is any integer greater than one. In some implementations, the grating structures 305 a-n may be grating peaks of a grating. In some other implementations, the grating structures 305 a-n may be grating valleys of a grating. The arrangement of 305 a, 305 b, . . . , and 305 n forms the grating in a grating region.

FIG. 3C shows an example of a view of a grating pattern 333 on a plane along the x-y dimensions. The descriptions of FIG. 3C may be applied to any one of the optical couplers described in this application. The grating pattern 333 includes an array of two-dimensional rectangular grating structures 307 a to 307 n along the x direction, and 307 a to 307 k along the y direction. In some implementations, the rectangular grating structure 307 a may be a grating peak of a grating. In some other implementations, the rectangular grating structure 307 a may be a grating valley of a grating. In some implementations, the rectangular grating structure 307 a may be composed of the same material as the layer 308, such as silicon. In some implementations, the rectangular grating structure 307 a may be composed of a different material from the layer 308. For example, the rectangular grating structure 307 a may be composed of silicon, and the layer 308 may be composed of InP. In some implementations, the rectangular grating structure 307 a may be a square or a non-square, or combinations of both structures. The arrangement of the rectangular grating structures 307 a-n and 307 a-k on the x-y plane forms the grating in a grating region. In some implementations, the period of the grating along the x direction 321 and period of the grating along the y direction 322 substantially match the interference pattern in the layer 308 along the x and y directions, respectively.

FIG. 3D shows an example of a view of a grating pattern 334 on a plane along the x-y dimensions. The descriptions of FIG. 3D may be applied to any one of the optical couplers described in this application. The grating pattern 334 includes an array of two-dimensional arbitrary-shaped grating structures 309 a to 309 n, where n is any integer greater than one. In some implementations, the arbitrary-shaped grating structure 309 a may be a grating peak of a grating. In some other implementations, the arbitrary-shaped grating structure 309 a may be a grating valley of a grating. In some implementations, the arbitrary-shaped grating structure 309 a may be composed of a different material from the layer 310. For example, the arbitrary-shaped grating structure 309 a may be composed of silicon-dioxide, and the layer 308 may be composed of silicon. In some implementations, the arbitrary-shaped grating structure 309 a may be a triangle or an ellipse or combinations of different shapes. The arrangement of the arbitrary-shaped grating structures 309 a-n on the x-y plane forms the grating in a grating region.

FIG. 3E shows an example of a view of a grating pattern 335 on a plane along the x-y dimensions. The descriptions of FIG. 3E may be applied to any one of the optical couplers described in this application. The grating pattern 335 includes an array of two-dimensional arbitrary-shaped grating structures 313 a to 313 n, where n is any integer greater than one. In some implementations, the shape of any one of the arbitrary-shaped grating structures 313 a to 313 n may be determined using numerical analysis. For example, a Finite-difference time-domain (FDTD) analysis program may be used to design the shape of each of the arbitrary-shaped structures 313 a to 313 n that optimizes the coupling efficiency. In some implementations, the distance between each one of the arbitrary-shaped grating structures 313 a to 313 n may be determined using numerical analysis. For example, a Finite-difference time-domain (FDTD) analysis program may be used to determine the distance between each one of the arbitrary-shaped structures 313 a to 313 n that optimizes the coupling efficiency. The arrangement of the arbitrary-shaped grating structures 313 a-n on the x-y plane forms the grating in a grating region.

In some implementations, the two-dimensional gratings shown in FIG. 3C, FIG. 3D, and FIG. 3E, can have lattice vectors (which define the unit cell size and shape) designed so that the locations of the in-phase antinodes of the interference region substantially match the locations of grating valleys and/or peaks.

FIG. 4A shows an example photonic integrated circuit 400 having a grating based optical coupler formed on a light source. The photonic integrated circuit 400 includes a light source region 430 configured to generate incident light. In some implementations, the light source region 430 may generate incoherent light. For example, a III-V quantum well laser diode may include one or more layers of active materials that generate incoherent light when pumped with electrical carriers. In some implementations, the incoherent light may be coupled to the interference region 410 through spontaneous emission. In some implementations, the light source region 430 may be confined at other surfaces except the surface coupled to the interference region 410.

The optical coupler includes a first reflector region 406, a second reflector region 414, an interference region 410, and a grating region 420. The structures of the first reflector region 406, the second reflector region 414, the interference region 410, the grating region 420, and the grating 418 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-3E. In some implementations, the interference region 410 and the grating region 420 is composed of silicon or III-V materials, the light source region is composed of III-V materials, and the first and second reflector regions 406 and 414 include DBR structures or metal coating.

The first reflector region 406 and the second reflector region 414 are configured to reflect incident light at a direction that is opposite to a propagation direction of the incident light as indicated by the arrow 434. The interference region 410 is formed between the first reflector region 406 and the second reflector region 414 and is coupled to the light source region 430. The interference region 410 may be configured to (i) guide the light generated by the light source region 430 to propagate along a first direction (the x direction in FIG. 4A), and (ii) confine interference light formed by light reflected between the first reflector region 406 and the second reflector region 414.

A portion of the light generated in the light source region 430 may be coupled to the interference region 410 through spontaneous emission or any other appropriate coupling mechanism. Light coupled to the interference region 410 may resonate along the x direction, as designated by the arrow 434. Similar to the operations described in FIG. 1A, the first reflector region 406 and the second reflector region 414 provide reflective surfaces that forms a cavity in the interference region 410, where a standing wave pattern may be formed. Since the interference region 410 has a fixed cavity length L_(Cavity), the standing wave pattern can only be formed by certain wavelengths, and the interference region 410 may therefore act as a wavelength filter. In some implementations, incoherent light generated by the light source region 430 may be transformed into coherent light in the interference region 410 by filtering out wavelengths that do not resonate in the interference region 410.

The grating region 420 includes a grating 418 formed on a region confining at least a portion of the interference light. The grating 418 is configured to emit a portion of the light at the z direction that is substantially perpendicular to the x direction. In some implementations, the grating 418 may be designed and fabricated in the grating region 420 to substantially match the standing wave pattern in the interference region 410. By matching the standing wave pattern, the grating 418 may act as an optical antenna and become the most efficient way for light to leave the interference region 410. Each grating structure may act to emit the light as a point wave, and all point-wave fronts emitted from individual grating structures are combined into a planar wave front which propagates in the z direction with low loss.

FIG. 4B shows an example photonic integrated circuit 401 having a grating based optical coupler where a light source region 431 is coupled to the interference region 411 by being embedded in the interference region 411. The light source region includes active material layers such as alternating layers of gallium arsenide (GaAs) and aluminum gallium arsenide (GaAlAs) or alternating layers of InGaAsP and InP. Any other combinations of active material layers forming a quantum dot, wire, and well structures that produce incoherent or coherent light are also within the scope of this disclosure.

The interference region 411 is formed between the first reflector region 416 and the second reflector region 424. The first reflector region 416 and the second reflector region 424 may be formed, for example, by coating a metal layer or dielectric layers on the end facets or by forming a DBR structure.

Compared to the descriptions of FIG. 4A where the light is generated outside the interference region 410, in FIG. 4B, the light is generated inside the interference region 411. The generated light resonates along the x direction between the first reflector region 416 and the second reflector region 424 in the interference region 411 to generate coherent light and form a standing wave pattern. The grating region 421 may be designed to substantially match the standing wave pattern, where the coherent light is emitted out of the photonic integrated circuit 401 through the grating region 421 along the z direction. In some implementations, the substrate 440 may be used as a supporting layer. In some implementations, the substrate 440 may include a DBR structure to further reduce the light that propagates in the −z direction.

FIG. 4C shows an example photonic integrated circuit 403 having a grating based optical coupler integrated with a light source, where the interference light can be controlled by a p-n junction. The photonic integrated circuit 403 includes a light source region 441, a p-doped region 442, an interference region 443, an n-doped region 444, a grating region 445, a first reflector region 446, and a second reflector region 448. The structures of the light source region 441, the p-doped region 442, the interference region 443, the n-doped region 444, the grating region 445, the first reflector region 446, and the second reflector region 448 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-3E.

Similar to the descriptions of FIG. 4A, incoherent light is generated in the light source region 441, where a portion of the light is coupled to the interference region 443. The coupled light resonates along the x direction between the first reflector region 446 and the second reflector region 448 in the interference region 443 to form a standing wave pattern and generate coherent light. A grating in the grating region 445 is designed to substantially match the standing wave pattern, and the coherent light may be emitted out of the photonic integrated circuit 403 through the grating 443 in the +z or −z direction, depending on the design of the grating 445.

In some implementations, the n-doped region 444 and a p-doped region 442 may be configured to provide an electric field in the interference region 443 with an application of a voltage or a current across the n-doped region 444 and the p-doped region 442. The interference region 443 may be configured to provide a different interference pattern with the application of a voltage or a current across the n-doped region 444 and the p-doped region 442 due to the generation, recombination, injection or depletion of free carriers. In the case where the interference pattern is changed due to the change of refractive index, the interference region 443 may stop lasing or may support another lasing wavelength. Therefore the application of a voltage or a current across the n-doped region 444 and the p-doped region 442 can act as a tunable wavelength lasing mechanism or a modulation to the coherent light.

FIG. 4D shows an example photonic integrated circuit 405 having a light source region coupled to an interference region through a waveguide region where a grating based optical coupler is configured to emit the generated light. The photonic integrated circuit 405 includes a light source region 450, a waveguide region 451, a first reflector region 452, a second reflector region 453, an interference region 454, and a grating region 455. The structures of the light source region 450, the waveguide region 451, the first reflector region 452, the second reflector region 453, the interference region 454, and the grating region 455 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-3E.

Here, incoherent light is generated in the light source region 450, where a portion of the light is coupled to the waveguide region 451. For example, the light source region 450 may include an indium phosphide (InP) quantum well (QW) structure bonded to a silicon waveguide, where the light generated by the QW is coupled to the silicon waveguide. The coupled light resonates between the first reflector region 452 and the second reflector region 453 in the interference region 454 to generate coherent light and form a standing wave pattern. For example, the waveguide structures of the first reflector region 452 and the second reflector region 453 may be designed to support a single mode propagation, which may filter out higher order modes in the interference region 454. A grating in the grating region 455 is designed to substantially match the standing wave pattern, and the coherent light is emitted out of the photonic integrated circuit 405 through the grating region 455 along a direction that is substantially perpendicular to the resonating direction of the light generated by the light source region 450. In some implementations, taper regions are used in the interference region to bridge a wider grating region and a narrower waveguide region. The narrow waveguide region may be used to suppress high order mode and the wide grating region may be used to match the external coupling device with different beam shapes areas and numerical aperture requirement. The descriptions of the taper regions may be applied to any one of the optical couplers described in this application.

FIG. 4E shows an example photonic integrated circuit 407 having a light source region coupled to a grating based optical coupler through the grating region 466. The photonic integrated circuit 407 includes a light source region 462, a first reflector region 478, a second reflector region 476, an interference region 472, a boundary 474, and a grating region 466. The structures of the light source region 462, the first reflector region 478, the second reflector region 476, the interference region 472, and the grating region 466 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-3E.

Here, incoherent light is generated in the light source region 462 and coupled to the interference region 472 through spontaneous emission or another suitable coupling mechanism. The coupled incoherent light is reflected by the reflector regions 476 and 478, and resonates along the x direction. The coherent light, when reaching lasing threshold, can emit either in the +z or −z direction depending on the design of the grating region 466. In some implementations, the grating region 466 can be designed to direct light mostly to the −z direction so that the emitted light does not couple back to the light source region 462.

In some implementations, the reflector 478 can be a partial reflector with less reflectivity than the reflector 476. The incoherent light generated in the light source region 462 can couple to the interference region 472 through spontaneous emission, and resonates along the direction 470. When reaching the lasing threshold, the coherent light can propagate in the −x direction through the boundary 474 into the partial reflector 478 and then into a waveguide, and at the same time propagate in the +z or −z direction into an external medium for further processing.

FIG. 4F shows an example of the photonic circuit 408 having similar layer stacks as FIG. 4B. The interference region 491 is composed of active materials such as III-V materials to generate incoherent light. The incoherent light resonates between the first reflector 486 and the second reflector 484 along the x direction and becomes coherent. The coherent light is emitted through the grating 492 upward since the DBR structure 494 is used to reduce the light that propagates in the downward direction.

FIG. 5A shows an example photonic integrated circuit 500 having an optical coupler integrated with a light detector. The photonic integrated circuit 500 includes an interference region 510, a first reflector region 506, a second reflector region 514, and a grating region 520. The interference region 510 includes a light absorption region that is configured to absorb at least a portion of the light in the interference region 510. The structures of the interference region 510, the first reflector region 506, the second reflector region 514, and the grating region 520 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-3E. The photonic integrated circuit 500 may be formed on a substrate 530.

The grating region 520 includes a grating 518. In some implementations, the grating 518 may be configured to receive, from an external medium, light propagating in the direction indicated by the arrow 536. Light may be coupled to the interference region 510 formed between the first reflector region 506 and the second reflector region 514. The interference region 510 is configured to confine interference light formed by light reflected by the first reflector region 506 and the second reflector region 514 along the direction indicated by the arrow 534.

The interference region 510 may be composed of a light absorption material that absorbs at least a portion of the interference light. For example, the interference region 510 may be composed of germanium that is configured to absorb light in telecommunications wavelengths. The absorbed light is converted to free carriers, which can be collected with an applied electrical field and used to determine an optical power level of the received light. In addition, light is reflected in a cavity formed by the first reflector region 506 and the second reflector region 514, providing an effective optical absorption. In some implementations, the interference region 510 includes silicon and/or germanium for light absorption and the grating region includes metal to create guided surface plasma modes.

FIG. 5B shows an example photonic integrated circuit 501 having a grating based optical coupler integrated with a light detector, where the light detection efficiency can be modulated by a p-n junction. The photonic integrated circuit 501 includes an interference region 541, a p-doped region 546, an n-doped region 545, a grating 542, a first reflector region 543, and a second reflector region 544.

Similar to the descriptions of FIG. 5A, light is received from an external medium, where a portion of the light is coupled to the interference region 541 through the grating 542. The coupled light resonates along the x direction between the first reflector region 543 and the second reflector region 544 in the interference region 541. The light absorbing material in the interference region 541 absorbs the interference light and coverts the light into free carriers.

In some implementations, the n-doped region 545 and a p-doped region 546 may be configured to provide an electric field in the interference region 541 with an application of a voltage or a current across the n-doped region 545 and the p-doped region 546. The absorption efficiency of the light absorbing material in the interference region 541 may vary based on the application of the voltage or a current across the n-doped region 444 and the p-doped region 442. Therefore the application of a voltage or a current across the n-doped region 444 and the p-doped region 442 acts as a configurable or tunable photo-detector.

FIG. 5C shows an example photonic integrated circuit 505 having a light absorption region coupled to an interference region through a waveguide region, where a grating based optical coupler is configured to receive light from an external medium. The photonic integrated circuit 505 includes a light absorption region 550, a waveguide region 551, a first reflector region 552, a second reflector region 553, an interference region 554, and a grating region 555. The structures of the light absorption region 550, the waveguide region 551, the first reflector region 552, the second reflector region 553, the interference region 554, and the grating region 555 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-3E.

Here, light is coupled to the grating region 555 from an external medium. The coupled light resonates between the first reflector region 552 and the second reflector region 553 in the interference region 554 to form a standing wave pattern. A light absorption region 550 is coupled to the waveguide region 551, and light is coupled from the waveguide region 551 to the light absorption region 550 for detection. For example, the light absorption region 550 may be a germanium photo-detector bonded on a silicon waveguide, where light propagating in the silicon waveguide is confined in the interference region 554 and the waveguide region 551, and evanescently coupled to the germanium photo-detector.

FIG. 6 shows an example of an optical coupler 600 integrated with p-n junctions. The optical coupler 600 includes a first reflector region 606, an interference region 620, and a second reflector region 614. The interference region 620 includes a grating region 630. The first reflector region 606, the interference region 620, the second reflector region 614, and the grating region 630 may be implemented using any one of the corresponding regions as described throughout the application.

The optical coupler 600 also includes p-n junction pairs including p-doped regions 621, 623, and 625, and n-doped regions 631, 633, and 635. In general, by controlling one or more p-n junction pairs, parameters such as output power and output wavelength may be actively controlled by the application of voltages or carriers injection. In some implementations, the p-n junction pairs 621/631, 623/633, and/or 625/635 may extend into the first reflector region 606, the interference region 620, and/or the second reflector region 614, respectively, for better controllability. In some implementations, the p-doped and n-doped regions may alternate to form interdigitated patterns or other patterns. The descriptions of the doped regions may be applied to any one of the optical couplers described in this application.

In some implementations, an n-doped region 631 and a p-doped region 621 may be configured to provide an electric field in the first reflector region 606 with an application of a voltage or a current across the n-doped region 631 and the p-doped region 621, where the first reflector region 606 may be configured to provide a different reflectivity with the application of the voltage or a current across the n-doped region 631 and the p-doped region 621.

In some implementations, an n-doped region 635 and a p-doped region 625 may be configured to provide an electric field in the second reflector region 614 with an application of a voltage or a current across the n-doped region 635 and the p-doped region 625, wherein the second reflector region 614 may be configured to provide a different reflectivity with the application of a voltage or a current across the n-doped region 635 and the p-doped region 625.

In some implementations, an n-doped region 633 and a p-doped region 623 may be configured to provide an electric field in the interference region 620 with an application of a voltage or a current across the n-doped region 633 and the p-doped region 623, wherein the interference region 620 may be configured to provide a different interference pattern for the interference light with the application of a voltage or a current across the n-doped region 633 and the p-doped region 623.

For example, by applying a reverse bias voltage, the electric field may extract the free carriers in a region, and therefore may modify the refractive index of the region. As another example, by applying a forward bias voltage, free carriers may be injected into a region, and therefore may modify the refractive index of the region.

FIG. 7A shows an example photonic integrated circuit 700 having multiple outputs. The photonic integrated circuit 700 includes a first waveguide region 702 configured to guide line in the direction as designated by the arrow 722. The photonic integrated circuit 700 includes a first grating region 720 formed on a side of the interference region 710. The photonic integrated circuit 700 includes a second grating region 721 formed on a different side of the interference region 710, for example, the opposite side as shown in FIG. 7A. The photonic integrated circuit 700 includes a reflector region 714, and may optionally include another reflector region 706. The photonic integrated circuit 700 includes a second waveguide region 728 that may be coupled to other passive and/or active optical components.

In some implementations, light from the first waveguide region 702 enters the interference region 710, and can be directed to a first external medium 730, a second external medium 732, or a second waveguide region 728. For example, similar to the descriptions of FIG. 6, an n-doped region and a p-doped region may be configured to provide an electric field in the interference region 710 with an application of a voltage or a current across the n-doped region and the p-doped region, where the portion of the light emitted in the +z direction and the portion of the light emitted in the −z direction may be controlled by the application of a voltage or a current across the n-doped region and the p-doped region. As another example, similar to the descriptions of FIG. 6, an n-doped region and a p-doped region may be configured to provide an electric field in the second reflector region 714 with an application of a voltage or a current across the n-doped region and the p-doped region. The reflectivity of the second reflector region 714 may be tuned, and light may be transmitted to the second waveguide region 728.

In some implementations, the light enters the interference region 710, and can be split into different portions of light that exit for the first external medium 730, the second external medium 732, and/or the second waveguide region 728. For example, the grating in the first grating region 720 may be designed such that the grating periodicity substantially matches the standing wave of a TE-polarized light. Similarly, the grating in the second grating region 721 may be designed such that the grating periodicity substantially matches the standing wave of a TM-polarized light. By controlling the proportion of TE and TM polarized light in the photonic integrated circuit 700, the portions of light exiting the photonic integrated circuit 700 for the first external medium 730 and the second external medium 732 may be controlled. The above example can serve as an effective polarization beam splitter.

In some implementations, a first layer 724 may be formed between the first grating region 720 and the first external medium 730. The first layer 724 may be formed to protect the photonic integrated circuit 700, or to provide a specific distance between the first grating region 720 and the first external medium 730. In some implementations, a second layer 716 may be formed between the second grating region 721 and the second external medium 732. The second layer 716 may be formed to protect the photonic integrated circuit 700, or to provide a specific distance between the second grating region 721 and the second external medium 732. For example, the first layer 724 may be a cladding, and the second layer 716 may be a substrate of the photonic integrated circuit 700. As another example, the first layer 724 may have a lower refractive index compared to the grating region 720.

FIG. 7B shows an example photonic integrated circuit 701 having multiple inputs and outputs. The photonic integrated circuit 701 may include a first waveguide region 751, a second waveguide region 752, a third waveguide region 753, a fourth waveguide region 754, p-n junctions 755-760 and 795-798, a first reflector region 761, a second reflector region 762, a third reflector region 791, a fourth reflector region 792, an interference region 770, and a two-dimensional grating 780. The structures of the first waveguide region 751, the second waveguide region 752, the third waveguide region 753, the fourth waveguide region 754, the p-n junctions 755-760 and 795-798, the first reflector region 761, the second reflector 762, the third reflector region 791, the fourth reflector region 792, the interference region 770, and the two-dimensional grating 780 may be implemented by any corresponding structures described throughout this application, for example, the corresponding structures in FIGS. 1A-6.

In some implementations, light from the first and the third waveguide region 751 and 753 enter the interference region 770, and may be directed to the second waveguide region 752, the fourth waveguide region 754, or out of the grating in the z direction as designated by the outward arrow 764. In some implementations, to minimize the back reflection of the input light from the first and the third waveguide regions, the reflectivity of the first and the third reflector regions 761 and 791 can be tuned during the initial design or through dynamic application of electrical field to match the one-circulation attenuation coefficients of the wave propagating along the x and y direction respectively.

In some implementations, light from the first waveguide region 751 enters the interference region 770, and may be split to different portions to the second waveguide region 752, the third waveguide region 753, the fourth waveguide region 754, and/or out of the grating in the z direction as designated by the outward arrow 764. For example, the grating region may include a two-dimensional grating 780 configured to separate light into two light portions propagating along two directions x and y according to wavelengths. As another example, the grating region may include a two-dimensional grating 780 configured to separate light into two light portions propagating along two directions x and y according to polarization. As another example, the two-dimensional grating 780 may be configured to reversely combine two light portions propagating along two directions x and y into one light portion.

In some implementations, the two-dimensional grating 780 may be configured to emit light upward in the z direction by combining two lights coming from two directions x and y, by substantially matching the periods of the grating 780 along the x and y direction to periods of the interference pattern along the x and y directions respectively. In some implementations, the two-dimensional grating 780 may be configured to emit light upward in the z direction by combining two lights coming from two directions x and y, by modifying the interference pattern along the x and y directions, through the application of electrical fields between p-doped and n-doped regions, to match the grating 780 pattern along the x and y directions respectively. As another example, the two-dimensional grating 780 may be configured to reversely split one light propagating at one direction (e.g. −z) into two light portions propagating along two directions x and y.

Similar to the descriptions of FIG. 7A, in some implementations, an n-doped region and a p-doped region may be configured to provide an electric field in the interference region with an application of a voltage or a current across the n-doped region and the p-doped region, where a respective proportion of each of the two light portions propagating along two directions is controlled by the application of a voltage or a current across the n-doped region and the p-doped region. For example, a voltage may be applied across the n-doped region 759 and the p-doped region 756 to control the routing or the splitting of the input light. In some implementations, one or more doped regions can be eliminated if there is no need to modulate the refractive index of corresponding regions. For example, the doped regions 756 and 759 can be eliminated if there is no need to dynamically modulate the interference region 770.

FIG. 8 shows an example of a photonic integrated circuit 800 suitable for packaging. The photonic integrated circuit 800 includes an edge emitting light source 801 that is butt-coupled to a waveguide 803. The light may be modulated by optionally including an integrated modulator 805 as shown in FIG. 8, or through direct modulation of the edge emitting light source 801. The modulated light is coupled to a grating-based optical coupler 807 as described in previous figures. Since light emitted by the optical coupler 807 may be designed to emit along a direction that is substantially perpendicular than the direction of light emitted by the edge emitting light source 801, the photonic integrated circuit 800 may be readily tested and packaged with a standardized optical equipment. For example, the die where the photonic integrated circuit 800 is fabricated on may be attached to the inside surface of a transistor outline (TO) metal can, where the grating may be aligned to the aperture of the TO metal can, allowing vertical transmission of light out of the package.

FIG. 9A shows an example of an optical apparatus 901, including a corner mirror 902 that provides a high-reflectivity. The descriptions of FIG. 9A may be applied to any one of the reflector regions disclosed in this application. In general, light propagates through the grating region 920, and is incident on the facets 931 a and 931 b of the corner mirror 902. Because the light is incident on the facets 931 a and 931 b beyond a total reflection angle, substantial light is reflected due to total reflection, achieving a high-reflectivity.

FIG. 9B shows an example of an optical apparatus 903, including a circular or elliptical facet 904 that provides a partial-reflectivity or a high-reflectivity. The descriptions of FIG. 9B may be applied to any one of the reflector regions disclosed in this application. In general, light propagates through the curved grating region 921, and is incident on the facet 904. In some implementations, the facet 904 may be coated with a metal layer to provide a high-reflectivity. The curved facet 904 refocuses the light back toward the direction of the waveguide region by the high-reflectivity curved facet 904.

FIG. 9C shows an example of an optical apparatus 905, including a distributed Bragg reflector (DBR) mirror 906 providing a high-reflectivity. The descriptions of FIG. 9C may be applied to any one of the reflector regions disclosed in this application. In general, light propagates through the grating region 922, and is incident on DBR mirror 906. In some implementations, the DBR mirror 906 may be designed to provide a high reflectivity for a range of wavelengths.

FIG. 9D shows an example of an optical apparatus 907, including a facet 908 providing a partial-reflectivity or a high-reflectivity. The descriptions of FIG. 9D may be applied to any one of the reflector regions disclosed in this application. In general, light propagates through the grating region 923, and is incident on the facet 908. Without any additional coating, the reflectivity may be determined using the Fresnel equations. In some implementations, the facet 908 may be coated with one or more layers of materials to increase the reflectivity. For example, the facet 908 may be coated with a metal layer to increase the reflectivity. As another example, the facet 908 may be coated with multiple dielectric layers to increase the reflectivity for a range of wavelengths. As another example, the facet 908 may be coated with a quarter-wavelength dielectric layer followed by a metal layer to increase the reflectivity for a range of wavelengths.

FIG. 9E shows an example of an optical apparatus 909, including a surface corrugated mirror 936. The descriptions of FIG. 9E may be applied to any one of the reflector regions disclosed in this application. In general, light propagates through the grating region 932, and is incident into the mirror 936 and gets reflected, forming an interference pattern.

Any other types of reflectors that can be integrated in a photonic integrated circuit may also be used as alternatives to the reflectors described in FIGS. 9A-9E. For example, a reflector region may alternatively include an anomalously dispersive mirror or a waveguide loop mirror.

FIG. 10 shows an example of a flow diagram for designing an optical coupler. The process 1000 may be performed by data processing apparatus, such as one or more computers.

The system obtains cavity dimensions (1002). In some implementations, based on the target light polarization/mode/wavelength/spot size, and the external medium (e.g., fiber on top of the grating or waveguide connected to the second side 114, etc.), the dimensions and materials of the cavity and the substrate can be determined. For example, for a single mode optical signal with center wavelength around 1310 nm, a Si layer cavity having a thickness of 250 nm on an oxide layer can be used. If the spot size of the external fiber is around 10 μm, the dimension of the cavity needs to be around or larger than 10 μm to allow the external fiber to be coupled onto the grating structure.

The system determines grating dimensions (1004). In general, the design of the grating dimensions depends on the standing wave pattern inside the cavity. The period of the grating pattern needs to be substantially matched to the standing wave pattern, and the other parameters, such as the duty cycle, and grating height, shape of the grating structure, are other design parameters that may be optimized based on parameters such as the incident light spot size, mode of the light, wavelength of the light, material of the cavity, intended emitting angle, etc. The standing wave pattern can be estimated assuming a specific mirror reflectivity.

The system determines grating characteristics (1006). For example, a numerical analysis tool such as a FDTD simulation system can be used to optimize the design parameters until a high directionality and the intended emission angle, for example a substantially vertical emission, are both reached.

The system determines mirror characteristics (1008). In some implementations, a numerical analysis tool such as a FDTD simulation system may optimize design parameters to generate a design for a back mirror having a reflection that is close to 100%. For example, the design may be a tapered waveguide distributed Bragg reflector (DBR), a waveguide loop mirror, or a Si—SiO2-metal coating layer.

In some implementations, the FDTD simulation system may be used to determine the one-circulation attenuation coefficient α, as described in FIG. 2. Based on the calculated one-circulation attenuation coefficient α, the system may design a front mirror having a reflectivity that substantially matches the one-circulation attenuation coefficient α. In some implementations, multiple doped regions can be placed near the regions to modify the properties of the regions through the application of electrical field.

In some implementations of the design flow, after obtaining the cavity dimensions (1002), a proper mirror design (e.g., tapered DBR or corner mirror or oxide-metal coating, etc.) can be determined to form the interference wave pattern. Then, grating structure on top of the interference region can be designed based on the initial interference wave pattern. By adding the grating structure, characteristics of the interference region may change, resulting in a modification of the interference wave pattern in the interference region. Therefore, some iteration processes might be needed for optimization. Based on the material quality and the physical dimensions of the interference region and the grating, the one circulation attenuation coefficient (α) can be determined along with the corresponding phase shift for the resonant condition. Then another mirror or reflector can be added on another side of the interference region.

FIG. 11 shows an example of a flow diagram for fabricating an optical coupler. The process 1100 may be performed by a system including data processing apparatus, such as one or more computers that control one or more apparatuses that perform the fabrication steps.

The system fabricates gratings (1102). The fabrication of gratings may be done by a combination of CMOS compatible fabrication techniques. For example, lithography techniques such as projection lithography, electronic-beam lithography, contact lithography, or any other suitable lithography techniques may be used to pattern the gratings. As another example, etching techniques such as dry etching, wet etching, or any other suitable etching techniques may be used to etch the patterned gratings. As another example, thin film deposition techniques such as chemical vapor deposition, plasma-enhanced chemical vapor deposition, sputtering, or any other suitable thin film deposition techniques may be used to deposit one or more layers of materials on the gratings.

The system can also optionally measure grating characteristics (1104) as a mid-process check point. In some implementations, the measurement of grating characteristics may be performed on a die level. In some implementations, the measurement of grating characteristics may be performed on a wafer level. In some implementations, input light is coupled into a photonic integrated circuit through a fabricated grating on the input end, and output light is coupled out of the photonic integrated circuit through another fabricated grating on the output end. In some implementations, input light is coupled into a photonic integrated circuit through a fabricated grating, where the light is reflected by a high-reflectivity reflector. The reflected light may be coupled out of the photonic integrated circuit through the same grating, and separated out by an optical circulator. The grating characteristics such as the one-circulation attenuation coefficient may be measured and analyzed by comparing the output light with the input light. In some implementations, the measurement of grating characteristics may be performed by shining a light at a tilt angle onto the fabricated grating and measure the reflected light at another angle.

The system fabricates mirrors (1106). In some implementations, after the system determines the one-circulation attenuation coefficient, the system can optimize the parameters for a reflector that matches the one-circulation attenuation coefficient. The fabrication of the one or more reflectors may be done by a combination of CMOS compatible fabrication techniques. In some implementations, the doped regions can be lithography defined, implanted and activated at locations near the regions needs to be modified through the application of electrical field.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, various forms of the flows shown above may be used, with steps re-ordered, added, or removed. For example, in FIG. 10, step 1008 can also be done before step 1004. As another example, in FIG. 11, step 1104 can also be done after step 1106.

Various implementations may have been discussed using two-dimensional cross-sections for easy description and illustration purpose. Nevertheless, the three-dimensional variations and derivations should also be included within the scope of the disclosure as long as there are corresponding two-dimensional cross-sections in the three-dimensional structures.

Embodiments and all of the functional operations described in this specification may be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments may be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer-readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable-medium may be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter affecting a machine-readable propagated signal, or a combination of one or more of them. The computer-readable medium may be a non-transitory computer-readable medium. The term “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus may include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. A propagated signal is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus.

A computer program (also known as a program, software, software application, script, or code) may be written in any form of programming language, including compiled or interpreted languages, and it may be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program may be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program may be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification may be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows may also be performed by, and apparatus may also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer may be embedded in another device, e.g., a tablet computer, a mobile telephone, a personal digital assistant (PDA), a mobile audio player, a Global Positioning System (GPS) receiver, to name just a few. Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory may be supplemented by, or incorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments may be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user may provide input to the computer. Other kinds of devices may be used to provide for interaction with a user as well; for example, feedback provided to the user may be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user may be received in any form, including acoustic, speech, or tactile input.

Embodiments may be implemented in a computing system that includes a back end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface or a Web browser through which a user may interact with an implementation of the techniques disclosed, or any combination of one or more such back end, middleware, or front end components. The components of the system may be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet.

The computing system may include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

While this specification contains many specifics, these should not be construed as limitations, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products.

Thus, particular embodiments have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims may be performed in a different order and still achieve desirable results. 

What is claimed is:
 1. An optical apparatus, comprising: a waveguide region configured to guide light propagating along a first direction; a reflector region configured to reflect incident light; an interference region formed between the waveguide region and the reflector region, the interference region configured to confine at least a portion of interference light formed by the incident light and the reflected incident light; and a grating region comprising a grating formed on a region confining at least a portion of the interference light, the grating configured to couple at least a portion of the light along a second direction that is different from the first direction.
 2. The optical apparatus of claim 1, wherein the second direction is substantially perpendicular to the first direction.
 3. The optical apparatus of claim 1, wherein the waveguide region comprises a second reflector region configured to (i) reflect, by a particular reflectivity, a portion of incident light, and (ii) transmit another portion of the incident light.
 4. The optical apparatus of claim 3, wherein light is attenuated based on an one-circulation attenuation coefficient after propagating one-circulation in the interference region, and wherein the reflectivity of the second reflector region is configured to substantially match the one-circulation attenuation coefficient.
 5. The optical apparatus of claim 3, further comprising: an n-doped region and a p-doped region configured to provide an electric field in the second reflector region with an application of a voltage or a current across the n-doped region and the p-doped region, wherein the second reflector region is configured to provide a different reflectivity with the application of a voltage or a current across the n-doped region and the p-doped region.
 6. The apparatus of claim 3 wherein a portion of the grating region is formed on the second reflector region.
 7. The optical apparatus of claim 1, further comprising: an n-doped region and a p-doped region configured to provide an electric field in the reflector region with an application of a voltage or a current across the n-doped region and the p-doped region, wherein the reflector region is configured to provide a different reflectivity with the application of a voltage or a current across the n-doped region and the p-doped region.
 8. The optical apparatus of claim 1, further comprising: an n-doped region and a p-doped region configured to provide an electric field in the interference region with an application of a voltage or a current across the n-doped region and the p-doped region, wherein the interference region is configured to provide a different interference pattern for the interference light with the application of a voltage or a current across the n-doped region and the p-doped region.
 9. The optical apparatus of claim 1, further comprising: a light source region configured to generate light, wherein the light source region is coupled to the interference region, the waveguide region, or the reflector region.
 10. The optical apparatus of claim 1, further comprising an optical package having an aperture above the grating region, the aperture configured to transmit light along the second direction.
 11. The optical apparatus of claim 1, further comprising a light absorption region configured to absorb at least a portion of light, wherein the light absorption region is coupled to the interference region, the waveguide region, or the reflector region.
 12. The optical apparatus of claim 1, wherein the grating region includes a two-dimensional grating configured to separate light into two light portions propagating along two directions, or combine two light portions propagating along two directions into one light portion.
 13. The optical apparatus of claim 1, wherein a second grating region is formed on a side of the interference region that is at a different side to the grating region, the second grating region configured to couple at least a portion of the light along a third direction.
 14. The apparatus of claim 1, wherein the reflector region includes one of a corner mirror, a DBR mirror, a dispersive mirror, a waveguide loop mirror, or a metal layer.
 15. The apparatus of claim 1, wherein a grating periodicity of the grating region substantially matches an interference periodicity of the light inside the interference region.
 16. The apparatus of claim 1, wherein the grating has lattice vectors formed so that the locations of the in-phase antinodes of the light inside the interference region substantially match the locations of the grating valleys or peaks.
 17. The apparatus of claim 1, wherein a portion of the grating region is formed on the interference region, the reflector region, or the waveguide region.
 18. An optical apparatus, comprising: a light source region configured to generate photons; a first reflector region and a second reflector region; an interference region formed between the first reflector region and the second reflector region and coupled to the light source region, and the interference region configured to (i) guide the incident light generated by the light source region to propagate along a first direction, and (ii) confine interference light formed by light reflected between the first reflector region and the second reflector region along the first direction; and a grating region comprising a grating formed on a region confining at least a portion of the interference light, the grating configured to emit a portion of the interference light along a second direction that is different from the first direction.
 19. The apparatus of claim 18, wherein the effective refractive index of the interference region is lower than the effective refractive index of the grating region.
 20. An optical apparatus, comprising: a first reflector region and a second reflector region; an interference region formed between the first reflector region and the second reflector region, the interference region configured to confine interference light formed by light reflected between the first reflector region and the second reflector region propagating along a first direction; and a light absorption region coupled to the interference region, the light absorption region configured to absorb at least a portion of the interference light; and a grating region comprising a grating formed on a region confining at least a portion of the interference light, the grating configured to receive, from an external medium, light propagating along a second direction that is different from the first direction.
 21. The apparatus of claim 20, wherein the light absorption region and the grating region have different material composition, wherein the grating region comprises metal forming surface plasmonic effect that couples light propagating along the second direction to the light absorption region. 